In known thermal cutting machines such as plasma cutting machines, molten material in the form of spatter (i.e., a splash of molten metal) flying from cut parts scatters in the surrounding area owing to the pressure of gas (e.g., plasma gas) ejected from the torch in the course of thermally cutting a workpiece. This not only adversely affects the work environment but also could cause fire hazard in the factory. Such spatter is generated, scattering into the environment particularly during a piercing process that is performed at a start of cutting a workpiece and mostly flies in a horizontal direction along the workpiece.
As an attempt to solve the above problem, there have been proposed and practically used various techniques for preventing the flying of spatter by shielding around the torch during thermal cutting. One example of such prior art techniques is disclosed in Patent Document 1. According to the technique of Patent Document 1, a hood, vertically movable by an actuator, is disposed around the torch and a command is issued to the actuator to move the hood down so as to cover the torch at least during the piercing phase. This prior art technique has the effect that, during piercing, the hood can be lowered with its lower end coming into close contact with the workpiece to thereby shut off spatter, whereas, during cutting, a desired clearance is provided between the lower end of the hood and the upper face of the workpiece, thereby preventing undesirable interference between the hood and the workpiece.
Patent Document 1: JP-A-2004-74254
FIGS. 10(a) and 10(b) show a partial plan view and partial enlarged view, respectively, of the plasma cutting machine described in Patent Document 1. The hood of this embodiment is open at one side. As illustrated in FIG. 10(a), the plasma cutting machine 50 has a cutting table 51 for supporting a workpiece and a dust collector (not shown) that is disposed under the cutting table 51, for collecting spatter, fume, exhaust gas, and the like generated in the process of cutting. Positioned above the cutting table 51 is a Y-axis carriage 52 movable in the direction of an X-axis that is one of two axes intersecting each other at right angles in the plane of the cutting table 51. Positioned above the Y-axis carriage 52 is a Z-axis carrier 53 movable in the direction of a Y-axis that is the other one of the two intersecting axes. The Z-axis carrier 53 includes a Z-axis movable pedestal (not shown) that is freely movable in a Z-axis direction perpendicular to the X-axis direction as well as to the Y-axis direction (i.e., the direction perpendicular to the plane of the drawing). Mounted on the Z-axis movable pedestal is a plasma torch 54 for jetting a plasma arc.
A hood (covering member) 55 is placed so as to enclose the outer periphery of a nozzle provided at the tip portion of the plasma torch 54. This hood 55 can be moved up and down in relation to the plasma torch 54 by a lifting and lowering mechanism. Concretely, the hood 55 is opened at a side opposed to the Y-axis carriage 52 and therefore has a substantially  (Japanese letter)-shaped section. The use of the hood 55 causes spatter to fly positively in a direction toward the open side and the flying spatter is received by a curtain-like, heat-resistant fabric 56 that is disposed in the Y-axis carriage 52 so as to extend over the travel range of the Z-axis carrier 53. In consequence, the spatter is prevented from flying and scattering into the environment.